Medical procedures often require locating and treating target areas within a patient. Radiation therapy and many surgical procedures require locating the target with a high degree of precision to limit collateral damage to healthy tissue around the target. It is particularly important to know or estimate the precise location of the target in radiation oncology because it is (a) desirable to accurately determine the accumulated dosage applied to the target and (b) detrimental to expose adjacent body parts to the radiation. In applications for treating prostate cancer, for example, it is detrimental to irradiate the colon, bladder or other neighboring body parts with the high-intensity radiation beam. Surgical applications, such as breast surgery and other procedures involving soft tissue, also require knowing the precise location of a target because a lesion in soft tissue is not necessarily fixed relative to external landmarks on the patient.
Many imaging systems have been used to locate areas or particular targets in a patient before performing radiation oncology or surgical procedures. Although x-ray, Magnetic Resonance Imaging (MRI), CT and other imaging techniques are useful to locate targets within the body at a pre-operative stage of a procedure, they are often not suitable or difficult to use in real time during surgery or radiation therapy. For example, the location of a lesion in soft tissue or in an organ may shift relative to external landmarks on the patient between the pre-operative imaging procedure and the actual radiation or surgical procedure. Additionally, when imaging systems are used during a radiation or surgical procedure, they may not provide sufficiently accurate measurements of the location of the lesions and they may interfere with the radiation or surgical procedure. Therefore, imaging techniques by themselves are generally not well suited for accurately identifying the actual location of a target for many medical applications.
Another technique to locate a target in a patient is to implant a marker relative to the target. Several types of tags or markers with resonating magnetic circuits have been developed to track feeding tubes, tag items, and mark tissue. For example, implantable markers that generate a signal have been proposed for use to locate a selected target in a patient in radiation oncology procedures. U.S. Pat. No. 6,385,482 B1 issued to Boksberger et al. discloses a device having an implanted emitter unit located inside or as close as possible to a target object, and a plurality of receiver units that are located outside of the patient. Boksberger discloses determining the location of the target object by energizing the emitter unit using a generator and sensing the signal from the emitter unit with the receiver units. Boksberger discloses and claims that the receiver units are configured to determine the gradient of the magnetic field generated by the emitter unit. Boksberger further discloses that the emitter unit is energized using a wired connection to the external generator. Boksberger also indicates that it is conceivable to use an emitter unit that is energized by a battery or excited by an electromagnetic field generated by the external generator. The wired device disclosed in Boksberger, however, may not be suitable for use in radiation oncology and many surgical procedures because it is impractical to leave a wired marker implanted in a patient for the period of time of such procedures (e.g., five to forty days). Moreover, Boksberger does not disclose or suggest anything with respect to providing an implantable emitter unit that is (a) suitable for use in radiation imaging processes or (b) compatible for use in magnetic resonance imaging devices after being implanted in a patient.
One challenge of using markers with resonating magnetic circuits is determining the relative location between the marker and the target so that the target can be tracked during a procedure or therapy. Accurately determining the location of the marker relative to the target is a precondition for accurately tracking the target based on the resonating magnetic field generated by the implanted marker. One reason that it is difficult to accurately determine the location of the marker relative to the target is that it can be difficult to identify magnetic resonating markers in radiographic images. The markers are difficult to see in radiographic images because (a) they should be very small so that they may be implanted for an extended period of time, and (b) they may not be sufficiently visible in high voltage radiation applications (i.e., megavolt radiation imaging). Moreover, even when a magnetic marker can be identified in an image, it can still be challenging to determine the orientation of the magnetic field generated by the marker relative to the target because it is often difficult to determine the orientation of the marker in the image. As such, implantable markers with resonating magnetic circuits may be difficult to use in radiation therapies and surgical procedures that require highly accurate localization of the target.